This PR is the outcome of a side discussion regarding issue #3360.

In short, in iris we've allowed the use of long_name from a coordinate to be used in the construction of the CellMethod, but that causes issues when the long_name contains one or more space (" ") characters, as it then quickly becomes ambiguous to parse the grammar suggested in the CF Conventions for a cell_methods attribute on loading after saving the CellMethod to NetCDF. That's not a fault of the CF Conventions, more our interpretation and implementation of it (from a file format agnostic point of view).

Reference, Cell Methods, 2nd paragraph:

In the specification of this attribute, name can be a dimension of the variable, a scalar coordinate variable, a valid standard name, or the word "area"

That said, this PR attempts to resolve this issue by ensuring that the coord_names and method used within the construction of a CellMethod are valid NetCDF name tokens, and thus can be parsed within the rules suggested in the CF Conventions without ambiguity (or over-engineering 😉).